[ci] Generate per-run SSH key in datakit-smoke workflow

Mirror the canary-ferry pattern: generate a fresh SSH key per run so
gcloud has a key to consume if SSH is needed (e.g. for future failure
diagnostics).

Author: ravwojdyla

.github/workflows/marin-datakit-smoke.yaml

@@ -52,6 +52,12 @@ jobs:
         with:
           project_id: ${{ secrets.GCP_PROJECT_ID }}
 
+      - name: Generate SSH key
+        run: |
+          mkdir -p ~/.ssh
+          ssh-keygen -t rsa -b 4096 -f ~/.ssh/google_compute_engine -N "" -q -C "gha-${{ github.run_id }}-${{ github.run_attempt }}"
+          chmod 600 ~/.ssh/google_compute_engine
+
       - name: Submit datakit smoke ferry
         id: submit
         shell: bash -l {0}